Web-Vet TM Neurology Specialists
Myasthenia Gravis
Immune-mediated myasthenia gravis (MG) is a relatively common neuromuscular disease affecting dogs and, occasionally, cats. Acquired MG has been observed in dogs older than 3 months of all breeds, but particularly in German Shepherd Dogs, Golden Retrievers, and Labrador Retrievers. In one report, the relative risk of acquired MG in different breeds of dog was highest in Akitas. Newfoundlands and Great Danes may be predisposed to a familial form of acquired MG. A bimodal age of onset (<5 years and >7 years) has also been reported in affected dogs, and spayed females may have heightened risk. In cats, Abyssinians and the closely related Somalis seem to be overrepresented and gender is not a risk factor in cats.
Clinical features and outcome of acquired myasthenia gravis in 94 dogs
Acquired myasthenia gravis (AMG) is 1 of the most commonly recognized neuromuscular diseases of the dog, characterized by the presence of autoantibodies (autoAbs) against nicotinic acetylcholine receptors (AChR) at the postsynaptic membrane. A new retrospective study based on 94 dogs with AMG has just been published as early view in JVIM. This multicenter study investigates the factors that predict clinical outcome and remission in a typical referral population of myasthenic dogs. The results of this study suggest that younger dogs presenting without regurgitation and with lower AChR Ab concentrations at the time of diagnosis experience better clinical outcomes in MG, including remission. Comorbid endocrine disease also appears to predict a better outcome.
On the right is a video of one of our previous patients diagnosed with AMG showing exercise-induced stiffness/weakness. Note the fatigue of the palpebral reflex as well as paw positioning response remaining intact on this dog which points to either junctionopathy or myopathy.
Classification of myasthenia gravis and congenital myasthenic syndromes in dogs and cats
This article carefully looks at the different aspects of MG, from the congenital form, in which immunity does not play a role, to the acquired form where autoimmunity against the neuromuscular junction is present.
Above is a video of a cat with MG. Compared to dogs, cats are not presented for so-called 'exercise-induced weakness' but rather for lethargy, reluctance to move, difficulty/reluctance to jump or go upstairs. Also, this video illustrates the best way, we feel, to do a neurological examination in cats while limiting stress.
Acquired myasthenia gravis (AMG) is an immune-mediated disorder in which antibodies are targeted against the nicotinic acetylcholine receptor (AChR) of skeletal muscle. This results in muscular weakness and excessive fatiguability. Several conditions have been associated with canine AMG, including other autoimmune disorders and neoplastic disorders. For the latter, AMG has been linked to paraneoplastic syndromes secondary to thymoma, osteosarcoma, cholangiocellular carcinoma, anal sac adenocarcinoma and cutaneous lymphoma. Cranial mediastinal masses and thymomas have been associated with AMG in humans, dogs and cats. Thymoma cells may express antigenic epitopes similar to those of nicotinic AChRs. The immune response to those epitopes results in the lack of functional AChRs in skeletal muscles. Surgical removal of hyperplastic thymic tissue is associated with long-term clinical improvement in humans with generalised forms of AMG. In this case report, a 9-year old Cockapoo with exercise-induced generalised weakness was diagnosed with AMG alongside with thymoma and cholangiocellular carcinoma. This dog was treated symptomatically with pyridostigmine bromide and complete surgical removal of thymoma and cholangiocellular carcinoma. This treatment resulted in resolution of the clinical signs and immune-remission as documented in this report by serial acetylcholine receptor antibody titers. Clinical remission was achieved after 8 month and immune remission was achieved after 18.5 months.
This study describes the outcomes of 8 cats diagnosed with acquired myasthenia gravis.
All cats had an excellent long-term outcome, achieving immune remission within 6 months of diagnosis, including 4 cats that did not receive any treatment and whose natural course of disease involved spontaneous remission. Clinical presentation was heterogeneous; skeletal muscle weakness and fatigue induced or exacerbated by the wheelbarrow exercise stress test were the most consistent. abnormalities associated with the presence of the clinical disorder.
Drop head syndrome (DHS), characterized by pronounced cervical ventroflexion, is a clinical syndrome that can be found associated with neuromuscular disorders, particularly myasthenia gravis. In this case series, two dogs are described, an 8-year-old Basset Hound and a 3-year-old Zwergpinscher, presenting with DHS as the main clinical sign. In both cases, a presumptive diagnosis of myasthenia gravis was made and were empirically treated with pyridostigmine. Telephone follow-ups at 6 and 4 months after diagnosis, respectively, were consistent with clinical remission. These cases highlight the diagnostic challenges of DHS, emphasizing the need for thorough evaluation to exclude numerous differential diagnoses. In cases where myasthenia gravis is strongly suspected despite negative tests, trial treatment with anti-cholinesterase drugs may be considered, with caution to avoid potential side effects.